Pyrolysis is the thermal transformation of carbon-containing starting materials, for example, biomass, into liquid pyrolysis concentrate (pyrolysis oil), solid pyrolysis coke and pyrolysis gas as pyrolysis products, and the method takes place under the exclusion of oxygen or at least essentially without the presence of oxygen. Pyrolysis is usually an endothermal process, however, individual sub-steps can also proceed exothermally. The percentage of the above-named pyrolysis products can be affected firstly by the choice of starting material (and in particular also due to its residual moisture) and secondly, by the prevailing process conditions, in particular the pyrolysis temperature, the pyrolysis time (dwell time) and by the rates of heating and cooling.
Frequently the heat consumption of a pyrolysis reaction can be supplied through combustion of pyrolysis coke and similar material, and/or through combustion of pyrolysis gas.
Thus pyrolysis represents a method in which, under specific conditions without the presence of oxygen and at temperatures between 270 and 1030° C., specific products such as gas, oil or charcoal can be produced for a broad range of applications. A distinction is made between fast pyrolysis (flash and fast pyrolysis) and slow pyrolysis, which is dependent essentially on the heating rate of the employed starting material.
The objective of fast pyrolysis is a maximum yield of liquid products. In this process, up to 45% liquid products are obtained. In this regard the employed biomass must be pyrolysed within seconds; the dwell time of the solid material in the hot zone is thus likewise within the range of seconds. Slow pyrolysis (also called carbonation) has been used for decades for the production of charcoal. The dwell time of the pyrolysis material in the pyrolysis zone here is in the range of hours to days. The heating rates are low in order to generate a maximum percentage of charcoal. In this method only starting material, in particular wood, with a moisture content of <25% is used; the method is not very well suited for other biomaterials as starting material.
An alternative to the above-named pyrolysis methods is pyrolysis in the medium temperature range with moderate dwell times. Document WO 2010/130988 A1 discloses one such method, in which the dwell time can amount to several minutes, depending on the particle diameter of the employed starting material. The pyrolysis here leads to products in which the tar content is reduced.
Document WO 2009/138757 A2 discloses a pyrolysis reactor in which the starting material is transported through the reactor by means of a worm conveyor and is simultaneously pyrolysed therein. At the end of the pyrolysis zone, the gaseous pyrolysis vapor is drawn off; the generated charcoal is carried back to the beginning of the pyrolysis zone via an external worm screw and is used as heat generator so as to obtain a better heat supply to the starting material to be pyrolysed. The ratio of charcoal to fresh starting material can be adjusted via the rotational speed of the outer worm screw; the dwell time in the pyrolysis zone can be adjusted here via the rotational speed of the internal worm screw.